Network of mid tapped transformers



1933. F w GAY 1,900,496

NETWORK OF MID TAPPED rI TRANSFORMERS filed Dec. 16, 1930 INVENTOR Patented Mar. 7, 1933 FRAZER W. GAY, OI NEWARK, NEW JERSEY NETWORK OF MID TAPPED TRANSFORMKERS Application led December 16, 1930i Serial Ne;l 502,735.

, This invention relates'toa novel method of controlling the output of a high capacity electrical supply station whereby very little impedance is offered to the normal expected flow of power from said substation, but a very great impedance is offered to the flow of power not in accordance with a predetermined plan'. ff

Heretofore it has beencustomary to insert reactors in all lines leaving a high capacity bus for two reasons. v

1st. To reduce the current that may fiow to a short circuit to a value such that a circuit breaker of reasonable size and moderate 5 cost may interrupt the flow of current to the short circuit.

2nd. To prevent so great a flow of current from the supply bus that the voltage of the supply bus will be reduced with the possibility that synchronous machines operating on said supply bus maybe thrown out of step.

In my novel method of distribution I employ a plurality of mid tapped auto transformers having a very large and predeter- 5 mined magnetizing current produced by either using a large air gap in the magnetic circuit or a magnetic circuit largely of non magnetic material. Several such auto transformers are employed in series.

The coils of the two halves of said auto transformers are well interlaced so that the drop in voltage on one half of the auto transformers is very little more than .the rise in voltage on the other half.

In the preferred form Vof the invention `a number of three phase feeders are grouped together on two three phase group busses.

' Each said group bus normally taking substantially the samek amount of powerv from the power source and at substantially the same power factor. Each corresponding phase of these group busses receives its power from opposite ends of anindividual one of a set of three auto transformers. These auto transformers each receive power at the mid points of the windings from a corresponding phase of the power system.

The exciting current taken by any auto transformer is so proportioned that the drop across the half winding that is carrying current to a fault is substantially equal to the drop in system voltage from the source of supply t'o its mid point, and since thedrop in voltage in the one half of such an auto transformer is very little more'thanvthe rise `in 55V voltage of the i other half, the utilization equipment connected to the non short circuited group bus receives substantially normal voltage during the fault.

In the `preferred form of the invention v shown' in the drawing, two sets of mid tapped auto transformers are used connected in series relation, and the magnetic circuits are so-designed that, for a three phase fault on one feeder, the current iowing to the fault through certain auto transformers willV cause the voltage to fall to 7 5% of normal at the mid taps of the first transformers in the series. These first auto transformers will introduce a further drop of 25% in voltage vto 70 a faulty current, so that approximately 50%' voltage will be applied at the mid taps of the auto transformers next to the fault.

In general each set'of auto transformers in the series through whichthe fault current, passes receives fault'current with a simultaneous incremental drop in voltage caused by the fault current, and each such set of auto transformers is designed to further'dr'opthe voltage of the fault current an equal incre` 80 ment, while it passes load currenttothe non short circuited' side of its windings increased in voltage substantially the same incremental amount whereby the load circuits are main# tained at normal voltage. Such a system of auto transformers in series constitutes in `effect a net work in which incremental voltage drops caused by a fault current flowing to a fault at one load point in the net work are compensated by other incremental volt# age rises whereby the voltage is maintained at substantially normal value at all load points other than the faulty load* point. 'y

An object of this invention is to provide a novel distribution system adapted tooperate olf a bus'of great capacity and which transfers voltage from a short circuited feeder to non short circuited feeders whereby the current to the non short circuited feeder is maintained at a value within the rupturing '100 v circuited lines at substantially normal value.

Y i A further object of this invention is to lprovidecurrent limiting means adapted to greatly limit the current iiowing to a faulty lineatl thetime of a fault, whilesaid means cooperate to introduce ,minimum-` impedance to the flow of normal currents flowing in accordance with a predetermined plan.

Other objects of lthis-invention not spe-1 cifically described will be understood by referring to the drawing in which Figure 1 is a diagrammatic representation. of siX three 'phase feeders receiving power fromk anhigh capacity,` three phase stationbus .through a plural-ity of f autotransformers. Figure-2 isa: one line representation of si-Xgfeeders receiving` power from a high` caf pacity sta-tion'v bus.k

- Figure 3 .isacne linerepresentation#of the; arrangementlshown inJFigure 1.

j, Figure,1 s`hows a diagrammatic repre# sentation of a stationbus-,oflgreat capacity distributing power to4 sixffeeders, in which Three-phase transformer banks` 7, 8', 9- rev ceiveppower from this busthroughccircuit breakers 4, 5, .6. A rlhe secondaries of these threetranstormer banks feed theirV induced .powerV to lthe siX feedersy through circuit breakers 1 0, y1.1 12.` Three autotransformers 22, 23, and 24l receive power at their mid points from ,transformer bank-I7 over wires 13,114, and 15.: Three autotransformers25, f

261,1 andk 27 receive power at their mid. points from transformer, hanlr'S. over4 wiresI 16,: 17 Vand 18. T hreetauto. transformers-28, 29,l and freceivepower from, transformer. bank 9 oxzervwines,1vf9,20, and, 2,1., `Autotransforin- Y ,e,rsj.22y 2 5, and. 28- have their endk lterminals connected ,together to. formt a, ,series circuit 22aands28toflead. 40g, 2 2 and, 25fto-f lead 43.; and.2l 5,and 28,11@ leadL46.. Likewise the, three auto;,transformersin another phase 2.3,.` l2.6,'

and 2 9?y are connectedto form.- a series, circuit and. theircommonpointsare connected. to

theleads4-1,A,4and: 4,71,K and autotransforme ers1 242'7 and.` 30 arev connectedto `forma series. Circuitzand theirl common. points are connectedy to,thewires,42 45, andf48.4 Wires 40,', 41s,. and., 42 ywhichreceivej power jointly from. transtermer banks 7-Y and 9-thrcugh one half, of,anto)transormerson-the one; handL 22., 2.324 androntthe other hand 28, r29, 30 are conneatedvv to the, mid points, of. anto. trans.- formers 31, 32andg 3.3,respectively.f'k In; the

' SameA manner wires-43, 44 and. 45`g7are, con:

nectedxto'themid points of auto transformers v34, 35,and 36,respectively- I Wires 46, 47, and48are connected: to the mid pointsof auto transformers;,37,i 38, and/39. Feeder wires 49, ,50,iand 51` connect .the- ,three phases Qfteeder w tothe terminals ofvone halfof thewindingsofauto transformers 31, 32,`and

33 through circuit breaker 52 and feeder wires Y windings of auto transformers 34, Y 35,

and 36 through circuit breaker 60 and feeder wires 61, 62, and 631 connect the` three phases of a fourth feeder' alto the other halves of thewindings vof yauto transformers 34,35, and

.36 throughcircuit breaker 64. In the same manner the three wires 65, 66, and 67 of a fifth feeder@ and jthegi three WiresV 69 70,771 ofi a six-th feeder k connect to the endl windf ingsv of auto transformers ,37, A38,andv 39 l :th-rough circuit, breakers 68 and 72respecY tively. L v y* c, 'Figure 2 shows aV one'line; diagram Vof a conventional substation in whichf repren Y sents a high voltage ous ofvvery greatcapacity; 7, 8, and 9V represent,transformer banks each ot l( ),00l);K"7A'capacityy and each having a'reactance at normal loa-dof 10%; Thesemay/be considered as thesame transformers shown in Figurel.v These three transforiners are4 shown feedingv ytheir Y in- K duced power-to a lowvoltage busrrepresented by the line ;y Six feeders @,b, c,r d, 6,17

each: of 5000 KVA capacity are represented b lvflines connected te bus 110-thr0i1g'h circuit breakers 52;,Y 56, 60,y 64, 68,and72,. .These may be consideredthe samev feeders shown'A in Figure 1.,.y 101',V 102,-v 103,1104'105.,. andi106 represent three phase` current limiting reactors in each feeder having-a reactance drop f of SJ/if/o ata-load of-r5000 KV 14. Y

A,threephase-fault is assumedvxtof come on:l feeder t as at X1. y Each transformer banlr'hasanaassumed,reactance of 10% at 1030.00 `KVLtlforf 5% at5000, KVA., and. the three banks in multiple' will have4 an im;- pedancefof: 5/3,orf12/,%" at 5000 KVA. l The total reactancev from 'bus-100.-to the fanlltrwill therefore; be, 1% plus 8-1/3k equals-10% andthe ance effects) will thereforebe-50,000 The; voltage on lbus-110 i; e. .they `-voltage; on

the otheriiwe f'eedersfvwill be 100% less 10 tinies;12/o,% orapproxmately 83.5%.

Figure 3f is a one line diagram of the ar:

ran-gement shown i-n Figure v1 in. whichline 101 vrepresentsl theathreebusses 1,235 122 represents. the three auto; transformers-22, 2B, and 24; 125 represents, thethree" auto tnansformers-25, 26, 27; and 128represents fics lil() KVA flowing-to theffauglt'(neglectingresistlows. If a three phase fault occurs say at X2 on feeder' a and the apparatus is designed to limit the current to the fault to a value 10 times the normal feeder capacity, or 50,000 KVA as in the previous example, then this current will be drawn through the left lhand windings of auto transformer 131 over wire 140 and substantially 1/2 or 25,000 KVA will be drawn through the left hand windings of alito transformer 122 and 1%), or 25,000 KVA A imum capacity and` at low power factor,

will be drawn through the right hand windings of auto transformer 128. One half (25,000 KVA) will thus come from bus 101 through transformer bank 7 and the other half (25,000 KVA) from bus 101 through transformer bank 9.

Since bank 7 and bank 9 each have 10% reactance to their rated load, i. e. 10,000 KVA they will each drop their voltage substantial- Y ly (neglecting resistance) when carryi ing 25,000 KVA at almost zero power factor provided, as we have assumed, the capacity of bus 101 is so great that its voltage is not materially reduced by the fault. 1t is proposed to design each of transformers 122, 125, 128 so that when one half of their respective windings carries 25,000 KVA, theL drop across this half of said windings is 25%, i. e. the drop across this half of said windings is the same as the drop in the power circuit up to the mid point of these windings i. e. in this case 25%. It follows therefore that the voltage on wires 140 will be 50% of normal. One half of this 50% voltage drop will be in transformer banks 7 and 9 and one half of the drop will be in one half of the transformer windings 122 and 128. Now if the windings in the two halves of auto transformers 122 and 128 are closely interlaced there will be a rise in voltage induced in the right hand windings of transformers 122 substantially equal to the drop in the left hand windings, and there will be a voltage rise induced in the left hand windings of transformer 128 substantially equal to thel drop in voltage in the right hand windings. It follows therefore that during the period of three phase fault at X2, the voltage on wires 1110 and the connected ends of the win dings of transformers 122 and 128 will be 50%. The voltage at the mid points of the windings of transformers 122 and 128 will be 7 5% of normal and the voltage on bus 101 will be 100%. The voltage on wires 143 and 146 and hence on feeders c, (l. c, and f will be 100%. Substantially 100% voltage will therefore be maintained on all the feeders except the faulty feeder a.

It follows from the above that for very heavy loads even up to a shert circuit on a single feeder, as feeder a in Figure 2 and feeder a Figure 3, the voltage regulation will be substantially the same whether protection is had by the conventional reactors of Figure 2 or by the auto transformers of the presentinvention Figurev 3. [However with a fault on a, Figure 2, feeders b, c, rife, and f will be subjectto a severe drop in voltage due to the fault, while feeders I), 0,' d, @,andV f, Figure 8, will have substantially normal voltage'maintained on ythem at the time of a fault 'onfeeder a. y f

A study of Figure 2 will show that when reactorsare permanently installed in. series and all feeders are uniformly loaded at maX- thereavill be a voltage drop in reactors 101 to 106 inclusive of substantially SI1/3%,

wliile'ifall feed-ers in Figure?) are uniformly loaded at rlow power factor vthe drop in the auto transformers maybe kept to less than 2% byy closelyk interlacingthe two halves of the windings. n

VAs many changes could be made in 'the above Aconstruction and many apparently Vwidely differentembodiments of this invenformers having its'mid-tap connected to a respective one. of said phase conductors forv receiving power therefrom, and six additional auto-transformers, said4 additional auto- I transformers havin gg substantially twice as much reactance ,to magnetizingy current as that possessed by said first named transformers, said additional auto-transformers having their mid-taps connected respectively to the respective winding terminals of said firstnamed auto-transformers.

i f 2.A Iii a distribution system,'in combination,

a three phase source of power having three phase conductors, three l mid-tapped autotransformers, each of said auto-,transformers having its mid-tap connected to a respective one of said phase conductors for receiving'A power therefrom, two groups of additional mid-tapped auto-transformers, each of said groups of `additional mid-tapped auto-transformers comprising three autotransformers,l each additional auto-transformer of one of sai d groups having its mid-tap connected to an ond terminal of one of said first-named autotransformers, and four substantially equal capacity load circuits, each of said load circuits being connected to three terminals of' one of said second named groups of midtapped auto-transformers whereby upon the occurrence of a fault on one of said load cir cuits, voltage drops caused thereby are conif to maintain substantially.. normal 4voltegge on :the,nonf au1ty1qad circuits. 1

,p3-,111; distribution systemince-mbinetion, mathfree, phase source of powerv having 5M threephase conductors, a, ,plurality of v,three :phase `:feeder ,.crcut's', fa,- network. 'of midytapped auto-transformers interconnecting said phase `,conductors tosaidieeclercrcuts, o each offsziid1,.aiuto-t1?ensfermersAleaving its mid-tapconnected to aoresypectveoneofsaid ,phase conductors],for, recevng power and its o fendtermineds eenneeteidftosaid feeder cir- Y cuits., said;` autoftransformers offering. relatively .littlempedenee ,tethepessageefpewer 11nif0-.rm1y,dstrbutedemong said feeder. eir- ,cu-its but ,oerng rlatvelygreat impedance Lto. the,,passagegoffcurremzs',to` e. fault on vone of said feeder circuitsv Wherebyvoltegefs maintained on,,themen-faultvfeeder Circuits- Line distribution SyStemtineeIbiI-letiem: a lthree phase source erpevverv having, ithree phaseJconduetQrs,@plurality '0f three r4,.pllasef. f feeder :rcujts7 ,at network .of mid- Vtztpped 4@ulterioransformers, eachof said autovtrztnsformers having its Inid-tap connected to a respective one of seidphase,conductorsnd. serving `to transmit 'power yfrom its enfiler- /rnnelsto said feeder circits,fsad autoftra-ns- .foljmers;.Cooperatingznponl the occurrenceof i eieultlon. Oneeisaidfeeder circuits, t0 meinteinnermel 'voltage on the ren-leiding feeder cireuits' .m I

K-Inv e., ,dstrbitiQn'Syetem, v inmblw- Ltio'rne*three.phase-,source of pevveiV having 3 flthree fpheee]eendutters,A @plurality Oiuthree -Aphase ffeeder circuits, la -network of midtapped atutoftrztnefermers, eh 'O''Sed auto- :transermersrid'said Syst-em offering e, reac- ...tanee toe. fault eurrent ovvrig'frem its mid- .tap tdoneteltmnal thereof; which Lreactance eisubstztntally ,equalte the retleteilee Of the .system from;the/sourceofenergy to'- the mide tap ofsad transformer. o'

, :111,teetim0ny, that I ,eldim'the invention Set y y Whavehereunto set my hand this 19thdey,ofDecembenleel. o t 

